Mantle In Eastern China Research Articles

P and S wave tomography of east-central China: insight into past and present mantle dynamics

East-central China has experienced multi-stage evolution among different tectonic units; thus the present geological structures are superimposed in a long history. But it is still unclear how different tectonic processes interact with each other, especially how the subsequent activity affects the previous structures. In this study, we determined new high-resolution P and S wave tomography of the upper mantle beneath the east-central China. We imaged the stagnant Pacific slab in the mantle transition zone and the hot low-velocity upper mantle in eastern China while cold high-velocity bodies beneath the Archean cratons in central China. More importantly, we newly revealed two important features. Firstly, a high-velocity zone is found in the uppermost mantle (50–150 km) beneath the Southeast China; its location is comparable to the distribution of the Cretaceous igneous rocks. Thus, we propose that the high-velocity anomalies reflect the remnant mafic rocks in the upper mantle after the volcanic eruption. Secondly, a high-velocity body is revealed in the deep upper mantle (300–500 km) beneath the southwestern Yangtze craton, possibly implying the delaminated cratonic lithosphere in the Early Mesozoic.

Crustal thickness beneath the Tanlu fault zone and its tectonic significance based on two-layer H-κ stacking

Tanlu fault zone (TLFZ) is the largest active fault zone in eastern China. It is characterized by complex tectonic evolution and multiple faults and marks the boundary between the North and South China blocks. An in-depth understanding of the distinct crustal structures of both parts of the TLFZ will provide valuable insights into the lithospheric and crustal thinning in eastern China, extensive magmatism since the Mesozoic, and formation mechanisms of metallogenic belts along the Yangtze River. In this study, a two-layer H-κ stacking approach was adopted to estimate the thicknesses of the sediment and crystalline crust as well as the corresponding νP/νS ratios based on high-quality teleseismic P-wave receiver functions recorded by permanent and temporary stations in and around the TLFZ. The geological units in the study region were delineated, especially the crustal structures beneath extensive sedimentary basins on both sides of the TLFZ. The following conclusions can be drawn: (1) The crustal thickness in and around the TLFZ greatly varies depending on the segment. In the northern segment, the crust is relatively thin beneath the eastern part of the Songliao Basin, a broad uplift of the Moho can be observed, and the Moho descends from south to north. The crust below the central and southern segments becomes thinner from west to east. The thickness of the crust is less than 30 km toward the eastern side of the boundary between the Jiangsu and Anhui provinces, that is, significantly thinner than in other areas. In terms of the νP/νS ratios, high anomalies were detected in the central-southern segments of the TLFZ, indicating the upwelling of deep mantle magma via deep faults. (2) Positive isostatic gravity anomalies were observed in the eastern part of the northern segment of the TLFZ and in the eastern part of the Suwan segment. The crustal thickness is smaller than that obtained from the Airy model of isostasy. This suggests that the lower crust in this area may have experienced intensive transformation processes, which may be related to crustal thinning (caused by crustal extension) and the strong uplift of the mantle in eastern China. The isostatic gravity anomalies between the eastern and western parts of the TLFZ indicate that the fault zone plays a dominant role in controlling the development of the deep crustal structure. (3) Significant crustal thinning was observed beneath the eastern part of the boundary between the Jiangsu and Anhui provinces in the southern segment of the TLFZ, suggesting that this area is prone to lithospheric thinning of the North China Craton. Due to the subduction, compression, and retreat of the Paleo-Pacific Plate during the Yanshanian Period as well as the dehydration of subducting oceanic crust (within subduction zones), the asthenosphere and oceanic crust in eastern China partially melted, resulting in mantle enrichment. The basic magma from the mantle is accumulated at the base of the crust, leading to magmatic underplating. In areas with weak topography toward the east of the TLFZ, magma rises to the upper crust and surface, resulting in the enrichment of multiple metal deposits in this area.

Differential exhumation of Jiaodong Peninsula during Early Cretaceous: Evidence from detrital zircon U–Pb isotopic dating

The significant Late Mesozoic thinning of lithospheric mantle in eastern China caused the formation of many basins and intensive volcanism. Crust exhumation process is a crucial part of supracrust response of lithosphere destruction. It has been studied in order to constrain the model of Late Mesozoic thinning of lithospheric mantle. The Sulu orogenic belt and the Jiaobei Terrane underwent a substantial exhumation process in Early Cretaceous, and though many studies have been carried out to investigate its exhumation history, it is still lacking understanding. Extensional terrestrial domain of the Jiaolai Basin was formed along with the Early Cretaceous exhumation of the Jiaobei Terrane and the Sulu orogenic belt. It provides an ideal research object for studying the exhumation process of Jiaodong Peninsula in Early Cretaceous because of the continuous sedimentary record in the basin. In this study, we analysed detrital zircon geochronology of three sandstone samples from Early Cretaceous sediments in north‐eastern part of the Jiaolai Basin. Detrital zircon age spectrums indicate that zircon ages consist mainly of six clusters, and they comprise Neoarchean ages, Palaeoproterozoic ages, Neoproterozoic ages, Triassic ages, Jurassic ages, and Early Cretaceous ages. The study shows that the Jiaobei Terrane and the Sulu orogenic belt served as source areas that provided detrital materials for Early Cretaceous strata in the Jiaolai Basin. Our results, along with those of previous studies, suggest that Jiaobei Terrane and the Sulu orogenic belt in Jiaodong Peninsula had discrepant exhumation history in Early Cretaceous.

Two Late Cretaceous A-type granites related to the Yingwuling W–Sn polymetallic mineralization in Guangdong province, South China: Implications for petrogenesis, geodynamic setting, and mineralization

Major and trace elements, whole rock Sr–Nd–Pb isotopes, LA–ICP–MS U–Pb zircon dating, zircon trace elements and Hf isotope data are reported for a suite of A-type granites from Yingwuling pluton in western Guangdong province, South China. Zircon U–Pb ages obtained by laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS) show that biotite granite and alkali feldspar granite were emplaced in 81.3±0.6Ma and 80.6±0.5Ma, respectively. Both of the two suites have the petrographic and geochemical characteristics of A-type granite. These granitic rocks are metaluminous to weakly peraluminous and have pronounced contents of total alkalis (Na2O+K2O=7.80–8.84%), Fe2O3T/MgO and Ga/Al ratios. They exhibit low MgO, CaO and TiO2 contents, enrichment in some LILEs and HFSEs (except for Zr, Eu and Y), depletion in Ba, Sr, P and Ti. They show A2 subtype affinity and were probably formed a temperature of ~800°C. The Yingwuling biotite granite has relatively high (87Sr/86Sr)i ratios of 0.70655 to 0.70928, low εNd(t) values of −5.8 to −4.2 and zircon εHf(t) values (−5.70–1.37). Whole-rock Nd isotopic and zircon Hf isotopic two-stages model ages mostly vary from 1057 to 1506Ma. The alkali feldspar granite display bulk rock εNd(t) values and (87Sr/86Sr)i ratios in the range of −6.6 to −6.1 and 0.70640 to 0.71077, respectively, and zircon εHf(t) values from −5.44 to 0.54, with Mesoproterozoic T2DM for both Nd and Hf isotopes. Geochemical and isotopic data indicate the Yingwuling A-type granitic magmas were drived from mantle–crust interaction. Zircon grains of Yingwuling granites have relatively low Eu/Eu* and Ce4+/Ce3+ ratios, indicating low oxygen fugacity. The visible tetrad effect in the Yingwuling granites indicates that it experienced strong fractionation and is close relationship to the W–Sn mineralization. Our new data together with previous published data indicate that Late Mesozoic A-type granitiods or alkaline intrusive rocks in South China can be grouped into four pulses: 195–180Ma, 180–149Ma, 136–116Ma and 110–79Ma. After 135Ma, with a change in the polarity of the subduction of paleo-Pacific plate, the crust and lithospheric mantle in eastern China became progressively thinned causing the upwelling of asthenosphere, which would have triggered crustal rocks to partially melt and generate Yingwuling A-type granitic magmas.

Mg-Sr isotopes of low-δ26Mg basalts tracing recycled carbonate species: Implication for the initial melting depth of the carbonated mantle in Eastern China

ABSTRACTRecent studies show that crustal carbonates recycled into the mantle can be traced using Mg isotopes of basalts. However, the species of recycled carbonates are poorly constrained. Carbonates have lower δ26Mg values and higher 87Sr/86Sr ratios relative to the mantle, but different carbonate species display different mixing curves with the mantle in the Mg-Sr isotopic diagram because of differences in their Sr and Mg contents. Thus a combined study of Mg-Sr isotopes can constrain the species of deeply recycled carbonates. Here, we present newly determined 87Sr/86Sr ratios of the <110 Ma basalts from Eastern China, and together with published Mg isotopic data we evaluate the species of recycled carbonates in the mantle and discuss their implication. The <110 Ma basalts display low δ26Mg values of ‒0.60 to ‒0.30‰ and relatively low initial 87Sr/86Sr ratios of 0.70328 to 0.70537, suggesting that their mantle source was hybridized by recycled carbonates with a light Mg isotopic composition which had more significant effects on Mg than Sr isotope ratios. Mg-Sr isotopic data indicate that the recycled carbonates consist of magnesite and aragonite, but the possibility of calcite and dolomite cannot be eliminated. Based on the carbonated peridotite solidus, the equilibrium line between dolomite and magnesite + aragonite, as well as the mantle adiabat, the initial melting depth of the carbonated mantle, the source region of the studied basalts, was constrained at ~300–360 km. Thus, the subducted depth of the west Pacific slab underlying the carbonated mantle and supplying recycled carbonates should be greater than ~300–360 km, consistent with the seismic tomography result that the west Pacific slab now stagnates in the mantle transition zone.

Methane-bearing melt inclusion in olivine phenocryst in Cenozoic alkaline basalt from Eastern China and its geological significance

Alkaline basalts originate from the partial melting of the upper mantle or deeper mantle region, and their study can contribute important knowledge of mantle composition. Melt inclusions in these basalts are small natural droplets trapped during the formation of olivine, and can be isolated from later alterations. Therefore, they can effectively record information from the early stages of magma formation and evolution. In this study, the volatile composition of 73 melt inclusions in olivine phenocrysts of alkaline basalts from Yangzhuang in Shandong Province were analyzed by Laser Raman Spectroscopy. The major and trace element compositions of some host olivines were measured by LA-ICP-MS. The results show that these melt inclusions contain amounts of methane (CH4) and other hydrocarbons, and some carbonaceous minerals such as graphite and carbonate. These findings prove that CH4 can stably exist in the upper-mantle environment of high pressure and temperature, and indicate that such a mantle region with low oxygen fugacity does exist below Eastern China. The host olivine phenocrysts are characterized by high Ni content and a high Fe/Mn ratio, implying the presence of a pyroxenite component in the mantle source. The subduction of the Western Pacific Plate beneath the lithospheric mantle of Eastern China is a possible source for the pyroxenite, combined with other geochemical and geophysical data. CH4 is an important form of deep carbon and the subduction of oceanic crust is responsible for the recycling of deep carbon.

Seismic anisotropy beneath the Chinese mainland

We investigated the upper mantle anisotropy beneath China by applying teleseismic shear wave splitting measurements at 119 seismic stations from CDSN and GSN/IRIS networks in China. The splitting observations are characterized by apparent diversity of anisotropy pattern in adjacent tectonic domains, including the Tianshan orogenic belt, Tibetan plateau, the Yangtze craton, the North China craton and northeastern region. In western China (Tianshan orogenic belt and Tibetan plateau), fast polarization directions of split SKS waves coincide strikingly well with the dominating trend of deformational crustal features and delay times range from 0.5 s to 1.6 s. While in eastern China, seismic anisotropy deduced from shear wave splitting reveals a homogeneous NW-SE trending structure, almost perpendicular to the strike of large-scale surface structures. The observed delay times of 1.5 s to more than 2 s favor consistent mantle flow over large mantle thicknesses. Based upon the straightforward relationships between seismic anisotropy and the development of lattice preferred orientation of mineral in upper mantle rocks, we interpret the splitting results in terms of tectonic fabric within the upper mantle. Since the lithosphere is less than 100 km thick beneath eastern China and the observed fast directions are subparallel to the trend of the absolute plate motion (APM) of Eurasian plate, we propose that the asthenosphere may mainly contribute to the anisotropic effects beneath eastern China. However, the upper mantle anisotropy beneath western China may have developed more recently in the subcrustal lithosphere because of rather small delay times and thick lithosphere. We also use the opportunity of the dense geodetic measurements available in China to discuss the coupling between the crust and mantle. In the Eurasia-fixed reference frame, GPS and shear wave splitting both depict a similar trend beneath eastern China, suggesting a lithospheric block “escaping” toward the east that could orient olivine [001] axis in the upper mantle. There is a strong coupling between the crust and the mantle in eastern China. A different behavior is observed in western China. The GPS vectors trend NS-NE in Tibet and NW in Tianshan, close to the regional compression direction, whereas the fast directions trend EW in Tibet and NE in Tianshan, suggesting a tectonic regime close to a mode of axial shortening, generating the development of EW-trending foliation in Tibet and NE-trending foliation in Tianshan at depth. The crust and mantle deform independently in western China.

Mafic dykes derived from Early Cretaceous depleted mantle beneath the Dabie orogenic belt: implications for changing lithosphere mantle beneath Eastern China

AbstractSHRIMP zircon U–Pb geochronological, elemental and Sr–Nd isotopic data from Early Cretaceous mafic dykes in North Dabie orogenic belt elucidate a change of Mesozoic lithospheric mantle in eastern China. The dykes are predominantly dolerite with the major mineral assemblage clinopyroxene + hornblende + plagioclase and yield a SHRIMP zircon U–Pb age of 111.6 ± 5.3 Ma. They have a narrow range of SiO2 from 46.16% to 49.78%, and relative low concentrations of K2O (1.07−2.62%), Na2O (2.45−3.54%), Al2O3 (13.04−14.07%), and P2O5 (0.42−0.55%) but relatively high concentration of MgO (5.94–6.61%) with Mg# 52–54. All the samples are characterized by enrichment of large ion lithophile elements (LILE, e.g., Ba, Th) and high field strength elements (HFSE, e.g., Nb, Ti). (87Sr/86Sr)i ratios from 0.704 to 0.705, εNd values from 3.36 to 4.33 and mantle‐depletion Nd model ages (T2DM) in the range 0.56–0.64 Ga indicate that the magma of the Baiyashan mafic dykes was derived from a young depleted mantle source. This finding is different from previous research on mafic dykes in the age range 120–138 Ma that revealed enrichment of LILE and depletion of HFSE, high initial Sr isotopic ratios and negative εNd, value which represents an old enriched mantle source. Ours is the first report of the existence of Early Cretaceous depleted mantle in eastern China and it implies that changing of enriched mantle to depleted mantle occurred at ca. 112 Ma, associated with back‐arc extension which resulted from the subduction of the Palaeo‐Pacific Plate towards the Asian Continent. Copyright © 2010 John Wiley &amp; Sons, Ltd.

Thermal and Tectonic Structures of the Crust and Upper Mantle in Eastern China

Thermal and Tectonic Structures of the Crust and Upper Mantle in Eastern China

Seismic anisotropy of upper mantle in eastern China

Based on the polarization analysis of teleseismic SKS waveform data recorded at 65 seismic stations which respectively involved in the permanent and temporary broadband seismograph networks deployed in eastern China, the SKS fast-wave direction and the delay time between the fast and slow shear waves at each station were determined by use of SC method and the stacking analysis method, and then the image of upper mantle anisotropy in eastern China was acquired. In the study region, from south to north, the fast-wave polarization directions are basically EW in South China, gradually clockwise rotate to NWW-SEE in North China, then to NW-SE in Northeast China. The delay time falls into the interval [0.41 s, 1.52 s]. Anisotropic characteristics in eastern China indicate that the upper mantle anisotropy is possibly caused by both the collision between the Indian and Eurasian Plates and the subduction from the Pacific and Philippine Sea Plates to the Eurasian Plate. The collision between two plates made the crust of western China thickening and uplifting and the material eastwards extruding, and then caused the upper mantle flow eastwards and southeastwards. The subduction of Pacific Plate and Philippine Sea Plate has resulted in the lithosphere and the asthenosphere deformation in eastern China, and made the alignment of upper mantle peridotite lattice parallel to the deformation direction. The fast-wave polarization direction is consistent with the direction of lithosphere extension and the GPS velocity direction, implying that the crust-upper mantle deformation is possibly a vertically coherent deformation.

Preliminary study on noble gas isotopes of mantle-derived peridotite xenoliths along the Tan-Lu Fault belt, Eastern China

In order to understand the nature of noble gas reservoirs beneath continents, elemental and isotopic compositions of noble gases were analyzed from spinel-lherzolites with anhydrous mineral assemblages (i.e., olivine+clinopyroxene+orthopyroxene +spinel±apatite±garnet); these are considered to be typical of the upper lithospheric mantle. Isotopic compositions of the noble gases were measured for peridotite xenoliths enclosed in Cenozoic alkaline basalts in eastern-central China to provide insight on mantle characteristics. The results show that the noble gas abundances and isotopic compositions in mantle-derived xenoliths from eastern-central China vary widely:4He abundances from 0.0024×10−6 to 25.0×10−6 (cm3stp/g);3He/4He ratios range between 0.33RA and 4.62 RA;40Ar/36Ar ratios from 250.56 to 1233.86;40Ar abundances from 0.97×10−6 to 34.18×10−6(cm3/g). Only three measured Ne isotopes show a degree of homogeneity:20Ne from 0.3 to 0.7 (cm3stp/g);20Ne/22Ne ratios from 10.5 to 11.0; and20Ne abundances are also rather uniform, from 35.8×10−13 to 47.8×10−13 (cm3stp/g), and129Xe/130Xe ratios from 6.50 to 6.70. It can be seen that3He/4He ratios vary largely, and40Ar/36Ar ratios are significantly lower than MORB and Proterozoic crust, but close to that of atmospheric values. On the basis of the noble gas abundances and isotope compositions, it can be concluded that Ne, Kr and Xe in mantle-derived xenoliths from NE China have atmospheric origin. The results also show the heterogeneity of noble gases in peridotite from the upper mantle in eastern China, which accords with the results of previous geochemical studies.

Flat mantle reflectors in Eastern China: possible evidence of lithospheric thinning

Recent 24 s deep seismic reflection records revealed five flat reflectors in the lithospheric mantle in Eastern China. With increasing depth, they are named M1 to M5 and can be seen on both field single-shot and stacked records. Reflector M1 corresponds to the Moho discontinuity, whereas M5 may be the reflection from the bottom of the current lithosphere, which is about 78 km deep according to geothermal measurements. The other three reflectors seem peculiar and might result from interactions between the lithosphere and deeper mantle. Based on lithological and geochemical data, it is suggested that the lithosphere has been thinned from about 150 km to about 60 km in the Late Mesozoic, and then has been thickened to about 78 km during the Cenozoic. The thinning process produced a granulite layer in the old lower crust caused by magmatic underplating, whereas an eclogite layer formed beneath owing to the subduction of the Paleo-Tethys and Yangtze Craton during the Permian and Early Mesozoic. Reflector M2 at about 12 s two-way traveltime (TWT) might result from the Paleozoic Moho, which represents the boundary between the previous granulite and eclogite facies. Reflector M3 at about 14 s might correspond to the bottom of the eclogite layer, beneath which the old lithospheric mantle remained. The old and the newly developed mantle may have different compositions, resulting in reflector M4. The multi-layered mantle reflectors demonstrate a mantle structure that possibly correlates with the lithospheric thinning process that occurred in Eastern China during the Late Mesozoic. The discovery of multi-layered mantle reflectors in the studied areas indicates a high heterogeneity of the upper mantle. Reflection seismology with improved technology, together with velocity and resistivity imaging and rock-physics measurements, can provide more details of the heterogeneity and related dynamic processes that occurred in the lithospheric mantle.

Gold Content of Mantle‐Derived Rocks from Eastern China and Its Implications

Abstract We have analyzed the gold content of 65 samples of mantle‐derived xenoliths and their host rocks from eastern China, which is found to be inhomogeneous, falling in the ranges of 1.0times10−9‐8.2times10−9 (averaging 3.8 times 10‐9) and 0.2times10−9‐5.3times10−9 (averaging 2.7times10−9) in the mantle‐derived xenoliths and the host basalts respectively. Except the samples from Wanquan County of Hebei Province and Anding County of Hainan Province, the gold content is the highest on the margins of the North China platform and decreases spatially towards the north and south, and temporally the samples of the Tertiary have a higher gold content than those of the Quaternary. The gold content of the mantle‐derived xenoliths and the host Paleozoic kimberlites is 3.8times10−9‐180times10−9 and 0.1times10−9‐38.0times10−9 respectively, which are higher than that in the mantle‐derived xenoliths in basalt and the host Cenozoic basalts. The mantle‐derived xenoliths have a higher gold content than the host rocks, but their relativity is not very clear, suggesting that the genetic relationship between them is complex. Our data indicate that the average gold content of the lithospheric mantle in eastern China is approximately equal to or approaching 3.8times10−9, which is lower than the average abundance of the upper mantle (5.0times10−9), higher than the average abundance of the continental crust (3.0times10−9) and only slightly higher than the average abundance of the lower continental crust (3.4times10−9). Except the samples from Hainan Province, the mantle‐derived xenoliths in basalts with a high gold content are distributed on the two margins of the North China platform, corresponding to the gold deposits‐concentrated districts in northwest Hebei Province and Shandong Province, showing the control by the old continental lithospheric mantle and the lower continental crust on the margins of the North China platform. The gold content of the mantle‐derived xenoliths from Hainan Province ranges from 7.1times10−9 to 15times10−9, suggesting that some blocks of the lithospheric mantle in Hainan Province may be derived from the ancient gold‐rich lithospheric mantle of western Australia and that Hainan Province may possibly be an important gold deposits‐concentrated district. Some mantle‐derived xenoliths in kimberlite of Shandong Province have an excessively high gold content amounting to 180times10−9, which provides new evidence for the possibility of “finding gold deposits near kimberlite pipes or alkali‐basalt volcanic vents”. Thererfore, the gold content of the mantle‐derived xenoliths may be an indicator to the distribution of gold deposits‐concentrated districts.

Layered Mantle Reflectors in Dabie‐Sulu Areas and their Interpretation

AbstractRecent 24s deep seismic reflection records revealed 5 flat reflectors in the lithospheric mantle in Eastern China. They are named M1 to M5 with increasing depth, and can be seen on both field single‐shot and stacked records. The reflector M1 corresponds to the Moho discontinuity, whilst M5 should be the reflection from the bottom of the current lithosphere that is about 78km deep according to geothermal measurements. The other three reflectors might result from interaction between the crust and mantle induced by the process of lithospheric thinning. Based on lithological and geochemical data, it is suggested that the lithosphere was thinned from about 150km to about 60km in the late Mesozoic, and then thickened to about 78km during the Cenozoic. The thinning process produced a granulite layer in the old lower crust caused by magmatic underplating, whilst an eclogite layer beneath occurred owing to the subduction of the Yangtze craton during the early Mesozoic. The reflector M2 at about 12s TWT might result from the Paleozoic Moho that turned out to be the boundary between the granulite and eclogite facies. The reflector M3 at about 14s might correspond to the bottom of the eclogite layer, beneath which the old lithospheric mantle remained. The old and the newly developed mantle may have different compositions, resulting in the reflector M4. The multi‐layered mantle reflectors demonstrate a typical mantle structure that correlates with the lithospheric thinning process that occurred in Eastern China during the later Mesozoic. As the idea of seismically transparent uppermost mantle has been widely accepted since the late 1980s, the discovery of the multilayered mantle reflectors in the Dabie‐Sulu areas of Eastern China implies that this idea can be challenged, suggesting that reflection seismology with improved technology can provide more details related to some dynamic processes occurring in the lithospheric mantle.

Mantle Redox State Evolution in Eastern China and Its Implications

Abstract Using the secondary spinel standard, the authors have precisely measured the Fe3+/Σ Fe values of spinels in mantle xenoliths from Cenozoic basalts in eastern China, and estimated the oxygen fugacities recorded by 63 mantle xenoliths through olivine‐orthopyroxene‐spinel oxygen barometry. The results indicate that the oxygen fugacities of the lithospheric mantle in eastern China are higher in the south than in the north. Among them, the oxygen fugacity of the North China craton lithospheric mantle is the lowest, similar to that of the oceanic mantle, while that of Northeast and South China are the same as that of the global continental mantle. The variations of mantle redox state in eastern China are mainly controlled by the C‐O‐H fluids derived from the asthenospheric mantle. According to the mantle oxidation state, it can be concluded that the C‐O‐H fluids in the lithospheric mantle of eastern China consist mainly of CO2 and minor H2O, but CH4‐rich fluids should come from the asthenosphere where the oxidation state is lower.

Redox state of amphibole-bearing mantle peridotite from Nüshan, Anhui Province in eastern China and its implications

Using secondary spinel standard method, we have measured precisely the compositions of spinels of amphibole-bearing mantle peridotite xenoliths from Nushan in eastern China, and calculated the mantle oxygen fugacities recorded by the xenoliths. Results indicate that the mantle metasomatism for forming amphiboles in Nushan region of Anhui has resulted in the decrease of mantle redox, which is in contrast with theoretical estimation and previous research results from other areas around the world. Combining with related studies on the mantle of eastern China, we give a reasonable explanation to the ‘new finding’ and further elucidate the compositions and nature of mantle fluids in eastern China.

Q β tomography under the crust and upper mantle in eastern China

On the basis of data of long period Rayleigh surface wave, we select 43 two-station paths which cover the eastern China thoroughly. By using the improved method of multi-filtration, we obtain the group velocity and amplitude spectrum, and then get attenuation factor for each paths. We employ Talentola inversion method to get local attenuation factor, and further invert the three-dimension Qβ image under the crust and upper mantle in the eastern Chinese continent. The Qβ image shows the following basic characters. There is correlation between the seismic activity and Qβ structure under the crust and upper mantle in North China region. The Yangtze block begins to collide with and subduct to the North China block from the southern border of the Qinling in the southern Shaanxi. In the large part of Yangtze quasi-platform appear an obvious high Qβ area at 88 km deep. In the east of Sichuan depression platform, the juncture of Sichun and Guizhou, and the Jiangnan block near the juncture of Guizhou and Hunan, the lateral variation of Qβ in the crust is little, and there is a high-Qβ layer no thinner than 40 km in the top mantle. In the Dian-Qian fold and fracture region between Yunnan and Guizhou, the vertical variation of Qβ at the region of the crust and upper mantle is little, there is a low-Qβ layer in the top mantle, about 40 km thick, low-Qβ layer of the upper mantle begins to appear at about 95 km deep. In the east of Yangtze quasi-platform and the central and eastern part of the South China fold system, the Moho is smooth, the lateral variation of Qβ in the crust is also little, low-Qβ layer of the upper mantle begins to appear at about 85 km deep.

The Thermal Structure of the Upper Mantle in Eastern China—Inferred from the Petrological Model

Abstract Cenozoic basalt in eastern China contains abundant ultramafic xenoliths which are specimens of pyrolites released during basaltic magma eruption. A total of 405 P‐T data of pyroxene in the ultramafic rocks have been collected, which present a more precise pyroxene geotherm. The average geothermal gradient in the upper mantle represented by the pyroxene geotherm is about 3.3°C / km, which is much less than that derived from the conductive thermal model (≡ 14°C / km), implying the great significance of convective heat transfer. The calculation shows that the contributions of convective and conductive heat transfers are 79% and 21%, respectively. The perturbation in the thermal structure of the upper mantle is an important manifestation of the tectonothermal event of Cenozoic continental rifting and intense basaltic volcanism in eastern China. Based on the pyroxene geotherm and its comparison with the current geothermal field derived from the measurements of the surface heat flows, it is suggested that the Moho may be a secondary thermal boundary. The current geothermal field and the thermal structure of the lithosphere in eastern China may mainly reflect the result of the tectonothermal disturbance in the Neogene‐Quaternary, in other words, the lithosphere has just begun to cool.

Mineral Chemistry of Clinopyroxenes in Mantle‐Derived Inclusions in Cenozoic Basalts from Eastern China and Their Geological Significance

Abstract In this paper, 183 chemical analyses of clinopyroxenes are collected from mantle‐derived inclusions in Cenozoic basalts of eastern China. Through synthetic studies of the relevant data, their mineral chemistry and geological significance are discussed. Based on the composition of clinopyroxenes the authors conclude that the Cenozoic upper mantle in eastern China is of the continental, primitive, metasomatic and high‐ temperature types and is related to rift structural environment and variation in composition of basalts. The geobarometry of clinopyroxenes indicates that Cenozoic basalts of eastern China are derived from the asthenosphere and that the average depth of faults progressively increases from south to north and from east to west.